1. Field of the Invention
The present invention relates to a black ink for ink-jet recording used in an ink-jet recording apparatus (e.g., a printer, a copier, a facsimile, or a word processor), and an ink set for ink-jet recording and an ink-jet recording method in which the black ink is used.
2. Description of the Related Art
The principle of an ink-jet recording method is that a liquid or fused ink is discharged through nozzles, slits, or a porous film to record images on a recording medium such as paper, a cloth, a film, or the like. Various methods of discharging an ink have been proposed, such as a so-called charge control method using an electrostatic induction force to discharge an ink; a method using vibrational pressure of a piezoelectric element to discharge an ink; and a so-called thermal ink-jet method forming and growing gas bubbles with heat to thereby generate pressure and then discharge an ink with the pressure, and any of these recording methods can provide an extremely precise image. Since an ink-jet recording apparatus is advantageous due to its compactness, inexpensiveness and silence, energetic studies have been conducted thereon, and many kinds of printers including not only monochrome printers but also color printers capable of recording full-color images, with which a good printing quality is obtained on plain paper such as report paper, copy paper, or the like, have been recently sold on the market, thereby attaining a large share in the field of recording apparatus.
The following characteristics are required for an ink-jet recording ink used in the ink-jet recording apparatus: (1) that the ink can provide uniform, bleeding-free and fog-free images with high resolution and high density on paper; (2) that the ink does not cause nozzle clogging which usually occurs due to drying of the ink at a distal end of a nozzle and that a good discharging responsiveness and a good discharging stability can be always obtained; (3) that the ink rapidly dries on paper; (4) that light resistance of the resultant image is good; and (5) that a long-term storage stability of the ink is good. In particular, with quickening of printing speeds, there is a need for an ink that rapidly dries even when applied to plain paper such as copy paper and that can provide a high image quality.
Ink-jet recording inks including dye as a colorant have been mainly employed, but such inks are poor in water resistance and light resistance. Therefore studies on inks including a pigment as a colorant have been conducted in order to solve such problems. Since a pigment is essentially insoluble in water, a method in which a dispersant is used in the ink has been studied as a method for stably dispersing the pigment in an ink. In the case where a surfactant is used as the dispersant, a surface tension of the resultant ink decreases to a greater extent than is necessary, easily causing degradation in printed image quality and non-printed gaps due to bubbles. In the case where a polymer dispersant is used, a viscosity of the ink tends to increase, and it is hard to achieve compatibility between ink-jet characteristics and dispersion stability. A common problem in methods in which a dispersant is used is that dispersion tends to become unstable due to interaction of a solvent, an additive or the like that is added to adjust ink characteristics, with the dispersant.
In view of such circumstances, a method has been being studied in which a dispersible pigment that has been made hydrophilic is used without using a dispersant. For example, Japanese Patent Application Laid-Open (JP-A) No. 8-3498 and Japanese Patent Application National Publication (Laid-Open) No. 10-510862 disclose methods and inks in which carbon black is made hydrophilic and used without a dispersant.
The methods for making a pigment hydrophilic solve problems caused by use of the above-described dispersant or improve ink properties by improving a hydrophilicity of surfaces of pigment particles to make the pigment self-dispersible in an aqueous medium. The use of such an ink enables ink characteristics close to those of an ink including a water-soluble dye which has been the main colorant employed for ink-jet recording ink.
In the case where ink-jet recording is adopted in an office, plain paper which is mainly used there includes a variety of products depending on the locality and availability. Therefore, it is necessary to obtain black images having satisfactory density and sufficient sharpness at edge portions thereof on such various kinds of paper.
Moreover, high-speed printing is needed from the viewpoint of better productivity in printing. However, in high-speed printing, the above-described black image quality, sufficient reduction in drying time of an ink, and, in the case where an interval between completion of monochrome printing and initiation of subsequent color printing or vice versa is short, prevention of bleeding of monochrome or color image portions has not been realized yet.
Therefore, there is a need for an ink-jet recording black ink which provides, on various kinds of plain paper, images having good black image density even if a drying time is short, and which does not easily bleed in boundaries between black image portions and color image portions when an interval between completion of monochrome printing and initiation of subsequent color printing or vice versa is short. Moreover, there is need for an ink-jet recording ink set and an ink-jet recording method using the ink-jet recording black ink.